This disclosure relates to methods and apparatus for measuring the load applied to a hydraulic cylinder by using a pressure sensor. The hydraulic cylinder may be part of a coiled tubing injector having a tubing guide. The load applied to the hydraulic cylinder may be used to indicate overload of the tubing guide.
When the load applied to the piston rod of the hydraulic cylinder is sufficient to overcome the pressure in the hydraulic cylinder, the piston moves away from the piston head (or the piston base) and compresses the hydraulic fluid in the chamber. The change in pressure is related to the load applied to the piston rod. The change in pressure can be detected using a pressure sensor connected to the cylinder.
False readings of the load applied to the piston rod may occur. Those false readings can be attributed to thermal expansion of the hydraulic fluid in the cylinder that causes an increase of the measured pressure, even in the absence of load increase applied to the piston rod. Thermal expansion may occur due to changes in the ambient temperature throughout the work day, or heating by other parts in contact with the hydraulic fluid.
Thus, there is a continuing need in the art for methods and apparatus for measuring the load applied to a hydraulic cylinder with a pressure sensor. These methods and apparatus preferably compensate for thermal expansion effects and improve readings of the load applied to the piston rod.